Boourns is not similar at all to boo, it is in fact the opposite of booing.

It comes from an episode of The Simpsons when Mr. Burns made a film for a film festival that no one in the audience liked, so they booed him. Burns, being surrounded by yes-men for his whole life was unfamiliar with being booed so Smithers told him they were chanting "boourns" rather than booing.

Mr. Burns stood up and asked the audience if they were booing or if they were chanting boourns. They responded by booing him, except for Hans Moleman who admitted to chanting "boourns" which leads one to the conclusion that Moleman liked the movie and was using the expression to show his enjoyment.

Idiot: "Dude, that movie sucked. Boourns!"
Other Guy: "You idiot, boourns is used for a good movie like Borat."